1. Field of the Invention
The present invention relates to a photosensitive recording medium and an image transfer system using such a medium, and more particularly to an image transfer technique utilizing photosensitive microcapsules, wherein a contrast of images formed on the medium may be adjusted with a minimum influence by a difference in photosensitivity of the microcapsules, a fluctuation in the intensity of a radiation to which the medium is exposed, or a fluctuation in developing conditions of the medium.
2. Discussion of the Prior Art
There is known a photosensitive recording medium having a multiplicity of microcapsules on one surface of its substrate. Each microcapsule includes a photosensitive resin, and a normally colorless chromogenic material which reacts with a developer material to produce a colored image spot, as a result of selective image-wise exposure of the medium and a subsequent developing process. The contrast of the formed images with respect to non-image areas of the medium is changed as a function of a difference in the exposure amount between the image and non-image areas. For example, such a photosensitive medium is disclosed in U.S. Pat. No. 4,399,209. The adjustment of the image contrast may be achieved by changing either the amount of exposure of the medium to an image-forming radiation, or the developing condition, such as a pressure by which the unexposed microcapsules are ruptured for image-forming reaction between the chromogenic and developer materials.
Conventionally, all microcapsules having the same chromogenic material for one color (including black) have the same photosensitive property, and therefore the image contrast adjustment is relatively difficult. More specifically, where the adjustment is accomplished by changing the amount of exposure of the medium to a radiation, this exposure amount must be adjusted within a relatively narrow photosensitive range of the microcapsules in which the density of the images formed on the medium is varied as a function of the exposure amount given to the medium. On the other hand, the intensity of the radiation generated by a light source or lamp is considerably time-wise fluctuated due to a variation in the temperature. Further, the amount of light generated per unit of the medium considerably differs locally along the length of the lamp. Thus, it is inevitably difficult to attain a precise adjustment of the contrast of the image areas formed on the medium, with respect to the non-image areas. In the case where the contrast adjustment is accomplished by changing the nip pressure of developing rollers to be exerted to the unruptured microcapsules on the medium, for example, the mechanism constructed for stable adjustment of the nip pressure tends to be complicated and large. Further, it is difficult to achieve a fine adjustment of the nip pressure over the entire length of the developing rollers.